Iodometric Determination of Vitamin C Triiodide, I 3 ,isamildoxidizingagentthatiswidelyusedinoxidation/reductiontitrations.
Triiodide is prepared by combining potassium iodide, KI, and potassium iodate, KIO3, in
acidicsolutionaccordingtothefollowingstoichiometry:
IO–3 +8I–+6H+3 I –3 +3H2O
(1)
In preparing triiodide, excess KI is used, so the concentration of I3‐ is determined by the
amountofKIO3addedtothesolution.Triiodidereactswithascorbicacid(vitaminC,amild
reducingagent)toformdehydroascorbateandthreeiodideionsaccordingtothereaction:
O
O
HO
O
HO
O
+ 3I- + 2H+
H2O + I3- +
(2)
OH
O
OH
HO
OH
O
Notice that one mole of iodine is consumed for each mole of ascorbic acid. In this
experiment, you will determine the amount of ascorbic acid in a vitamin pill using the
triiodidereactionina“backtitration”.Afterextractingtheascorbicacidfromvitaminpills
withacid,youwillconvertittodehydroascorbateusingaknownexcessoftriiodide.The
amount of triiodide remaining after reaction 2 will be determined by titration of the
triiodidewithastandardizedthiosulfatesolution.Notethatyoudonottitratetheanalyte
directly,butrathertitrateanaddedreagentafterexcesshasbeenadded.Thisisknownasa
backtitration.Thebacktitrationreactionis
(3)
I3‐+2S2O32‐3I‐+S4O62‐
Note that 2 moles of thiosulfate are consumed for each mole of triiodide present. The
endpoint is determined using a starch indicator. Mixtures of starch and triiodide have a
deepvioletcolor,butwhenthetriiodideisconsumedthesolutionbecomescolorless.Over
timethestarch‐triiodidecomplexcanstabilize,anditbecomesdifficulttoreduceallofthe
triiodide.Thereforeitispreferabletoaddthestarchjustbeforetheendpoint.Fortunately
thetriiodidesolutionitselfhasayellow‐to‐browncolor,dependingonconcentration.When
thesolutionturnspaleyellow,youknowthatmostofthetriiodidehasbeenconsumed,and
youareneartheendpoint.Thenyoucanaddthestarchindicator.YouknowhowmuchI3‐is
addedtothevitaminsample,andwiththetitrationresultsyoucandeterminehowmuchis
leftaftertheoxidationofascorbate.Thedifferencebetweentheseistheamountoftriiodide
consumedintheoxidationofascorbate,whichisrelatedtotheamountofvitaminCpresent
inthesamplebythestoichiometryofreaction2.
IodometricDeterminationofVitaminC
Procedure
PreparationofReagents
1. Starch indicator will be provided 2. Solid potassium iodide will be available 3. 0.3 M H2SO4 will be available 4. ~0.04 M Sodium thiosulfate solution will be provided. You should be able to complete the experiment with 250 mL of this solution. 5. Preparation of 0.01M KIO3 Solution: a. Accurately weigh approximately 0.535 g of solid reagent and record the mass to 4 decimal places. b. Deliver the KIO3 to a 250 mL volumetric flask and add 100 mL of DI water. Swirl to dissolve, then dilute to volume. c. Compute the molarity of the solution. (FW = 214.00) Thissolutionisusedforbothsteps;donotdomorethan4titrationswithoutconsulting
withyourTA
Standardizing the Thiosulfate Solution
1. Collect 250 mL of the thiosulfate solution. You must use the same solution for the entire experiment. 2. Pipet 25.00 mL of the KIO3 solution into each of 3 Erlenmeyer flasks. Using the calculated KIO3 solution concentration, calculate the volume of titrant (thiosulfate) required assuming that the thiosulfate concentration is 0.04 M. This gives the approximate endpoint. 3. Add 1 g of KI and 20 mL of 0.3 M sulfuric acid solution to each flask. 4. Titrate the triiodide with the thiosulfate solution until the brown solution becomes pale yellow. Then add 2 mL of the starch indicator solution and titrate until the violet color of the starch‐iodine complex just disappears. This is the endpoint. 5. Repeat this procedure for a total of three precise titrations. DONOTdomorethanfour(4)titrationswithoutconsultingwithyourTA
AnalyzingtheVitaminC
1. Weigh a sufficient number of vitamin tablets so that approximately 500 mg of ascorbic acid is obtained (normally one tablet – your TA may tell you how many tablets to use). Grind the tablets with a mortar and pestle. 2. Record the weight of the resulting powder that is actually analyzed. Then transfer the known mass of powder to a 250 mL volumetric flask. IodometricDeterminationofVitaminC
3. Add 100 mL of 0.3 M sulfuric acid. Swirl the flask for about 10 minutes, and then let it stand for several minutes. Swirl again, and then dilute to the mark with 0.3 M sulfuric acid. Because of the fillers and binders used in vitamin tablets, your solution may be cloudy – this is normal. 4. Deliver 25.00 mL of the vitamin C solution to an Erlenmeyer flask. 5. Add 1 g of solid KI and 25.00 mL of standardized KIO3 to the flask. Titrate the remaining triiodide with the standardized thiosulfate solution as above, taking care to add the starch solution just before the end point. 6. Repeat this titration twice for a total of three precise determinations. 7. Calculate the average mass of vitamin C in each tablet and the uncertainty in the determination, as indicated on the report sheet. IodometricDeterminationofVitaminC
Student Name: ____________________________
Chemistry 3200 Iodometric Determination of Vitamin C Date: _________
LabInstructor: ______________
Section: ________
MassofKIO3: ________________
MolarityofKIO3solutionbymass: ________________
Volumeofthiosulfatesolutionusedforstandardization:
_________________
_________________
_________________
Averagevolumeoftitrant: __________  __________
Molarityofthiosulfatesolution: __________  __________
MassoftheVitaminCtablet(s): ________________
MassofVitaminCpowderinvolumetricflask: ________________
Totaltabletmass(ifdifferent): ________________
Volumetitrantusedineachdetermination:
_________________
_________________
_________________
Averagevolumeoftitrantused:
__________

__________
Molesofascorbicacidtitrated:
__________

__________
Gramsofascorbicacidtitrated:
Masspercentofascorbicacidin
pill:
Massofascorbicacidinpill:
__________

__________
__________

__________
__________

__________
Iodometric Determination of Vitamin C
Student Name: ____________________________
Calculation for the standardization of the thiosulfate:
Calculation for the percent of vitamin C per tablet:
Calculation for error analysis (Include a list of the errors and their
sources):
Iodometric Determination of Vitamin C